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Vascular plants without seeds (The Megaphyll Line of Evolution:…
Vascular plants without seeds
The Microphyll Line of Evolution: Lycophytes:-
Lycophytes represent a distict line of evolution out of the early land plants that resembled
zosterophyllophyte
Morphology: Earliest lycophytes were members of the genera
Drepanophycus and Baragwanathia similar to their presumed ancestor, the zosterophullophytes.
Heterospory: It is a necessary precondition for the evolution
of seeds
Extant Genera
Selaginella: Less common in temperate North America, and its
plants are smaller and easily overlooked to be mosses. They can be distinguished from lucopodiums by a small flap of tissue, the
ligule
Lycopodium: Common in forests from tropical regions to the
arctic , are small herbs with prostrate rhizomes that have true roots and short upright branches.
The Term "vascular Cryptogams": They have
vascular tissue and that because they lack seeds their reproduction is hidden
Early Vascular Plants
Rhyniophytes: Plants of Cooksonia had an epidermis with a
cuticle, a cortex of parenchyma, and a simple bundle of
xylem composed tracheids with annular secondary walls.
Plants of Cooksonia were homosporous; there were no separate microspores and megaspores
Zosterophyllophytes: They were small herbs without
secondary growth. Many features are similar to rhyniophytes but three character differentiate them. Their sporangia were
lateral; not terminal; sporangia opened transversely along the top edge, and their xylem was an exarch prostele
Xylem Structure of Early Vascular Plants
Exarch protostele: Metaxylem located in the centre of the
xylem mass and protoxylem on the edges as several groups next to the phloem
Siphonostele: this is not the evolved one which pith is present
in the center as occurs in the stems of ferns and seed plants
Endarch Protostele: Protoxylem is located in the centre and
metaxylem differentiates on the outer edge of the xylem mass
The Megaphyll Line of Evolution: Euphyllophytes
Equisetophytes: Classified as division Arthrophyta, consist of
several genera of extinct plants and one genus, Equisetum, with 15 extant species called Horsetails
Eusporangia and Leptosporangia
Eusporangium: Initiated when several surface sells undergo
periclinal divisions, resulting in a small multilayered plate of cells Outer cells develop into the sporangium wall, and the inner cells
proliferate into sporogenous tissue
Leptosporangia: Initiated when a single surface cell divides
periclinally and forms a small outward protrusion, undergo several more divisions, which result in a small cell of
sporogenous cells and thin covering of sterile cells; only few spores are produced
Euphyllophytes are united by three synapomorphies
They have megaphylls
They have a 30-kilo base inversion in the large single copy
region of their plastid DNA
Their roots have exarch xylem
Psilotum and Tmesipteris: These two small genera contain
the simplest of all living vascular plants
Monilophytes: They are plants we know as ferns
Ferns: Early ferns apperared in the Devonian Period and
then diversified greatly
